The invention relates to rotary drill bits for use in drilling deep holes in subsurface formations, and of the kind comprising a bit body having a leading face and a gauge region, a plurality of cutting elements mounted at the surface of the leading face of the bit body, a passage in the bit body for supplying drilling fluid to a plurality of openings in the leading face of the bit body, and at least one junk slot in the gauge region of the bit body, whereby drilling fluid emerging from said openings flows over the leading face of the bit body and past cutting elements thereon, so as to cool and clean the cutting elements before exiting through said junk slot.
The invention is particularly, but not exclusively, applicable to drill bits of this kind in which the cutting elements are in the form of preforms of polycrystalline diamond spaced apart over the leading face of the bit. Such preforms may be mounted directly on the bit body or mounted on studs which are received in sockets in the bit body. This invention relates, however, primarily to the cooling and cleaning of the cutting elements and is thus also applicable generally to bits using other types of shaped manufactured cutting elements.
The cutting elements will normally be spaced over the leading face of the bit at various distances from its axis of rotation, from positions close to the axis to positions close to the gauge region. Conventionally, the openings in the surface of the bit body to which drilling fluid is supplied are normally located fairly close to the axis of rotation of the bit so that drilling fluid emerging from the openings flows outwardly over all parts of the leading face of the bit to the junk slots at the outer periphery. In some cases, however, auxiliary openings may be provided at locations intermediate the axis of rotation and periphery to augment the flow of fluid across the outer parts of the bit. Channels may be formed in the leading face of the bit extending away from the axis of rotation thereof to direct the flow of drilling fluid. For example, the cutting elements may be mounted on blades extending away from the axis of rotation of the bit, the blades defining between them channels for the drilling fluid and the cutting elements being so mounted on the blades that fluid passing outwardly along the channels flows over the cutting elements so as to cool and clean them. The openings for drilling fluid in the surface of the bit are often in the form of nozzles inserted in sockets in the bit body.
The cooling and cleaning of the cutting elements by the drilling fluid is, in conventional bits of this type, most efficient in the vicinity of the openings which are in the region close to the axis of rotation of the bit. This is due to the high velocity of flow and turbulence of the drilling fluid as it emerges from the openings. The drilling fluid spreads out as it passes outwardly away from the axis of rotation of the bit, however, and its velocity and turbulence therefore decrease with the result that the cooling and cleaning effect of the fluid on the cutting elements located towards the outer periphery of the bit is greatly reduced. The less efficient cooling of the outer cutting elements, and the consequent higher temperatures to which they are thus submitted, can lead to early failure of these cutting elements, rendering the bit unusable for further drilling in spite of the fact that a large proportion of the cutting elements nearer the axis of rotation of the bit may still be in good condition. The present invention therefore sets out to provide a drill bit construction in which the cooling and cleaning of cutting elements near the outer periphery of the drill bit is improved.